In some industrial applications, steel tubing is subjected to severe stresses, and where a series of lengths of pipe or tubing have to be coupled together, the connection or coupling itself must be able to bear the applied stresses. For example, for use as oil well casing, such tubing may be subjected to thermal stresses imparted by steam injection into an oil or gas well where temperatures of the order of 650.degree. F. may be reached. In the field, thermal stresses may subject the tubing to compressive and tensile axial loads approaching or conceivably even exceeding the actual yield strength of the material in the pipe body. Thus, any connection or coupling for joining together successive lengths of pipe must be able to withstand the axial loading without failure and still be resistant to leakage from internal pressures. As the pipe is alternately heated and cooled, the axial loading on the pipe and couplings may become alternately compressive and tensile, and throughout the coupling must maintain its seal with the pipe ends in resisting internal pressure.
Such couplings (connections) comprise a male and mating female coupling component. The male component is a suitably configured threaded portion at one end of the steel pipe, constituting the pin member of the coupling. A mating annular female component long enough in the axial direction to receive the pin ends of two adjoining lengths of pipe is internally configured and threaded at each end for mating engagement with the pin member, thereby completing the coupling. The annular female element is often referred to as the box element or box member of the coupling. Alternatively, one end of the pipe could be upset and internally threaded to constitute the female component of the connection.
Conventionally, the pin (male) member of the coupling or connection is tapered inwardly from the proximal end of the threaded portion to the distal end to mate with a similarly tapered female threaded member of the coupling. The taper facilitates entry of the pin member into the box member.
In pipe couplings, a seal is typically maintained. The seal may be effected between the mating threaded portions of the pin member and the box member of the coupling, but this kind of seal is subject to ready leakage. In other couplings, some auxiliary sealing element (e.g. an annular elastomer) is provided. In yet other couplings for use in which the pin member of the present invention is a species, the axial load-bearing threaded portions of the coupling do not themselves necessarily provide a seal (although they may do so); the seal relied upon is a separate metal-to-metal seal provided adjacent the axial load-bearing threaded portions, in both the pin (male) and the box (female) members of the coupling or the connection.
The threaded axial load-bearing portion of the coupling (or connection) should conform to certain known design principles. The total axial bearing surface provided by the full-depth load-bearing threads should be at least equal to the cross-sectional area of the pipe material. The angle of orientation of the stab flanks of the axial load-bearing threads should differ from the angle of orientation of the load flanks by not less than about 15 degrees. Excessive tilt of the distal end of the pin that causes the yield strength of the steel to be exceeded is generally to be avoided. Other general principles of thread design and coupling design will be known to those skilled in such design work and should be applied to the design of the connection of the present invention.
Various types of connections are known in which a metal-to-metal seal is provided. The designer may choose to provide on such surfaces a controlled degree of roughness, as by shot peening, grit blasting, glass bead peening, or helical microgroove threads having a pitch very small relative to the pitch of the loadbearing coupling threads.
U.S. Pat. No. 5,498,035, granted Mar. 12, 1996, describes a pipe connection whose male and female components are matingly threaded for coupling engagement, and each of which is provided with a frusto-conical sealing surface adjacent the load threads. The sealing surface is provided with microgroves or other shallow fine surface variations. The slope of the frusto-conical surface of the sealing area of the male component is slightly mismatched with that of the sealing area of the female component. This slight mismatch enables the sealing surfaces to engage one another with a bearing force that resembles that of a shrunk-fit circular cylindrical sealing arrangement with force peaks at each end of the sealing area and a sealing force greater than designed minimum in between the ends. This sealing characteristic enables the seal to be maintained despite possible damage to one end of the sealing area. The seal is designed for use with a high temperature graphite particle-containing lubricant with a relatively high content of solid graphic particles that can be mashed into the sealing surfaces when the connection is made up. A number of other preferred design characteristics of the connection are described in more detail in U.S. Pat. No. 5,498,035, incorporated herein by reference.
Although the connection described in U.S. Pat. No. 5,498,035 is a very effective connection, problems can sometimes arise if the connection is manufactured with some dimensions departing from optimum dimensions by close to maximum permitted tolerance. In particular, at the distal end of the pin (male) component of the connection, the available material for machining can, depending upon the particular nominal dimensions of a pin for certain pipe sizes, be relatively thin, rendering machining somewhat difficult, and tending to produce out-of-tolerance pins. Further, if the material yields too readily, there is a risk that the sealing efficacy of the connection may be impaired. Further, thinner material tends to be less resistant to occasional blows; a greater amount of damage can be done to the pin under a blow of given force if the material is thin.